Coated implants and methods of coating

ABSTRACT

A method including forming a first coating layer derived from an alkoxide on a substrate having a dimension suitable for an implant and forming a second coating layer on the first coating layer that promotes osseointegration. An apparatus comprising a substrate having a dimension suitable as a medical or dental implant and a coating on a surface of a first coating layer derived from an alkoxide and a second coating layer that promotes osseointegration.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/360,144, filed Feb. 22, 2006 (now U.S. Pat. No. 7,341,756), which isa continuation of U.S. patent application Ser. No. 10/454,406, filedJun. 4, 2003 (now U.S. Pat. No. 7,067,169).

This invention was made with United States Government support undercontract 1R43DE14927-01 awarded by NIH/NIDCR. The Government has certainrights in this invention.

BACKGROUND

1. Field

Medical/dental implants.

2. Background

Metal implants are widely used in medical and dental applications, suchas in orthopedic hip and knee surgeries and in dental surgery. Over twomillion orthopedic procedures and over 10 million dental implantprocedures are performed in the United States every year. Implants failbecause of poor osseointegration between the implant and the naturalbone. Implants are typically made of metal materials, with titanium (Ti)and its alloys being favored due to their biocompatibility andmechanical properties. For the implants to function successfully, adirect chemical bond between the implant and the bone needs to formrapidly and needs to be retained over many years while the implant isloaded. Metal materials, however, do not form a direct chemical bondwith bone. In order to promote osseointegration between the metalimplant and bone, a layer of osseointegration promotion material isincorporated on the implant. Calcium phosphate ceramic materials are anexample of coating materials that promote osseointegration. The mostpopular coating among the calcium phosphate family is hydroxyapatite(HA) due to its chemical stability and osteoconductivity.

Important parameters in the long-term behavior of implants coated withHA include at least acceptable coating-substrate bond strength andbiostability (i.e., a low dissolution rate of the coating). In order toimprove coating-substrate (usually a metal) bond strength and otherproperties, a variety of coating techniques have been explored todevelop thin (generally less than 10 microns) coatings of HA and othercalcium phosphates. U.S. Pat. No. 4,908,030 discloses a method offorming a thin HA coating on an implant using ion beam sputtering. U.S.Pat. No. 5,817,326 discloses a method in which one or more layers of HAsol-gel are cured to densify on a titanium alloy implant, followed by anon-line-of-sight ion implantation process, in order to strengthen theadhesion of the HA coating to the substrate. U.S. Pat. No. 5,543,019discloses a method of forming a thin coating layer on the surface of animplant using a plasma sputtering process. Other methods developedinclude pulsed laser deposition and magnetron sputtering.

Another approach to improve the bonding capability of an HA coating ontoa metallic substrate has been the deposition of a composite coating,wherein a metallic phase is introduced to serve as either anintermediate layer or a second (continuous or dispersed) phase in an HAmatrix. For example, Dasarathy et al., in “Hydroxyapatite/metalcomposite coatings formed by electrocodeposition,” J. Biomed. Mater.Res., 31, 81-89 (1996), describes an electro-codeposition process tocoat a cobalt/HA (Co/HA) composite coating on a Ti substrate with a bondstrength up to 37 MPa. Using plasma spray technique, Brossa et al., in“Adhesion properties of plasma sprayed hydroxyapatite coatings fororthopaedic prostheses,” Bio-Med. Mater. Eng., 3, 127-136 (1993), andNakashima et al., in “Hydroxyapatite coating on titanium-sprayedtitanium implant,” in Bioceramics 6, P. Ducheyne and D. Christiansen(eds.), Butterworth-Heinemann, Oxford, 1993, pp. 449-453, describes adouble-layer coating including an HA layer on top of a porous Ti precoaton a Ti substrate. This double-layered coating was shown to outperform amonolithic HA coating in adhesion properties. German patent “Coating ofimplants,” Gruner, Heiko (Plasmainevent A.-G.) Ger. Offen. DE 3,516,411(Cl. C23C4/04) Nov. 12, 1986, describes a multi-layered coatingcomprising a Ti precoat, a Ti/HA composite layer and an HA overlayerformed by plasma deposition. The multi-layer coated implants show fastand stable fusion between the coated implant and the bone. On Ti-6Al-4Vsubstrate Ferraris et al., in “Vacuum plasma spray deposition oftitanium particle/glass-ceramic matrix biocomposites,” J. Am. Ceram.Soc., 79, 1515-1520 (1996), plasma-sprayed a Ti particle-reinforcedbioactive glass composite coating, which exhibited a higher bondstrength than that of monolithic bioactive glass coating.

Pure titanium implants have become a preferred choice instead of calciumphosphate coated implants in recent years because of the criticaldisadvantages of previous calcium phosphate coatings. Plasma sprayingand sputter coating are two major techniques that were widely used forHA coatings. These methods tend to have a problem with the dissolutionof calcium phosphate, at a 50 percent rate, through a high temperatureprocessing. Different phases of non-HA phosphate calcium sprayed onimplants are easy to dissolute in body solution. The calcium phosphatecoated implant by these methods also failed in long term stability oftendue to fracture at the coating-titanium interface which appeared to havebeen caused by the poor adherence of the HA film to the metal substrate.Furthermore, sputter plasma and coating tend to produce a non-uniformcoating when they are applied to an irregular or porous surface.

Accordingly, a need exists for an implant with an improved bond strengthand biostability as well as a method of forming such implants for use inorthopedic and dental applications. In addition, a majority ofcommercially available titanium implant systems utilize some degree ofmacroporous surface topography based on preclinical and clinical datathat a roughened surface topography appears to enhance the rate andspeed of functional implant-tissue integration. A conformer coating onthe rough implant surface is needed.

SUMMARY

A method is described. In one embodiment, the method is suitable forcoating biocompatible, biostable, and bone conductive material of metalimplant surfaces. An exemplary method includes forming a first coatinglayer on a portion of a substrate (e.g., a metal material) having adimension suitable for an implant (e.g., a medical or dental implant),and forming a second coating layer on the first coating layer, thesecond coating layer, including a material having a property thatpromotes osseointegration.

For a multiple layer coating such as described, the first coating layermay be a single molecular layer that can react with a metal implantsurface to promote or achieve chemical bonding or other adherence. Inone embodiment, the first coating layer is made from alkoxides (such asalkoxysilanes) tri-functional silanes that tend to form a chemical bondto the metal implant. The reacted derivative may include a positivelycharged ligand on a surface of the formed layer or film. The secondcoating layer may be a layer of calcium phosphate material, such asnanosized hydroxyapatite particles, that may bond (e.g., ionically bond)or otherwise adhere to the positively charged first coating layer. Inone embodiment, the second coating layer is made from a negativelycharged hydroxyapatite (HA) nanoparticle, colloidal solution. Negativecharged hydroxyapatite crystalline nanoparticles tend to form arelatively strong bond to the positively-charged first coating layerthrough ionic bonding forces. the attraction to the positively-chargedfirst coating layer may also produce a negative charge at the surface ofthe second coating layer. Methods to immobilize bone inducing growthfactors onto bone conductive hydroxyapatite that accelerates thefixation of the implant to the bone are also described.

An apparatus is further described. In one embodiment, an apparatusincludes a substrate having a dimension suitable as an implant for usein a medical or dental application. The substrate may be a metalmaterial such as titanium, tantalum, cobalt, chromium, and theirrespective alloys. Since it may be desirable, in one application toinsert or embed (implant) the substrate into bone material, thesubstrate includes a surface coating over a portion thereof,representatively, at least the portion intended or designed to be incontact with bone or other tissue. In one embodiment, the surfacecoating includes at least two coating layers: a first coating layerhaving a property such that it will bond or otherwise adhere to thesubstrate, particularly a metal substrate; and a second coating layer onthe first coating layer having a property that promotes osseointegrationbetween the apparatus and bone or other tissue. Bone inducing growthfactors may also be added, possibly to the second coating layer.

Additional features, embodiments, and benefits will be evident in viewof the figures and detailed description presented herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and advantages of embodiments will become morethoroughly apparent from the following detailed description, appendedclaims, and accompanying drawings in which:

FIG. 1 schematically illustrates a cross-sectional side view of aportion of a substrate having multiple coating layers on a surface.

FIG. 2 shows a RT-PCR evaluation of steady state mRNA levels of type Icollagen, osteopontin, osetocalcin, TO1, TO2 and TO3 in rat bone marrowstromal cell cultured on Ti disks with turned, turned-HA, acid-etched,acid-etched-HA surfaces. A housekeeping gene, GAPDH was used as aninternal control.

FIG. 3 shows an implant push-in test at day 14 of in vivoosseointegration in a rat femur. (n=6).

DETAILED DESCRIPTION

A coated apparatus and a method of coating an apparatus is disclosed. Inone embodiment, a suitable apparatus is a medical/dental implant. Animplant in this context means a device intended to be placed within ahuman body such as to connect skeletal structures (e.g., a hip implant)or to serve as a fixture for a body party (e.g., a fixture for anartificial tooth). FIG. 1 shows a medical/dental apparatus includingsubstrate 10 of a metal material. A suitable metal material for amedical/dental implant includes, but is not limited to, a titanium, atantalum, a cobalt, a chromium, or their respective alloys. One suitablealloy for titanium is a titanium (Ti)-aluminum (Al)-vanadium (V) alloy(e.g., Ti-6% Al-4% V). Substrate 10 includes surface 15 (top surface asviewed) that may or may not have an irregular topography or may beporous (e.g., macroporous).

Referring to FIG. 1, overlying a portion of surface 15 of substrate 10is first coating layer 20, of, for example, a material having a propertysuch that it will bond or otherwise associate or adhere to substrate 10.Suitable materials for first coating layer include, but are not limitedto, materials derived from alkoxides. Representatively, first coatinglayer 20 is derived from an alkoxide having the general formula:YR₂M(OR₁)_(x-1)

where M is one of silicon (Si), titanium (Ti), boron (B), aluminum (Al),zirconium or other ceramic types of metals;

where R₁ is an alkyl moiety such as methyl or ethyl;

where R₂ is an organic moiety, such as an alkyl (e.g., methyl, ethyl),pyridin-benzene, benzene;

where Y is a positively charged moiety, such as an amine (NH₂) moiety ormetal (e.g., Fe, Ca, Mg); and

where x is the valence state of M.

One suitable material for use in forming first coating layer 20 is amultifunctional silane. Representatively, a suitable multifunctionalalkoxysilane is a trifunctional silane, aminopropyl-trimethoxysilane(APS). APS tends to bond to a metal substrate through the alkoxy groupsand provide a positively-charged surface through the amine-bondedligand. (substrate-O—Si—R—NH⁺). An example of another positively chargedligand (YR₂) that may be suitable is pyridin-Fe.

In one embodiment, first coating layer 20 is a single molecular layer (amonolayer) having, in this illustration, positively charged end ligandsat a surface of the coating (the surface opposite substrate 10). Firstcoating layer 20 may be formed on a portion of a surface of substrateincluding the entire surface. Suitable methods for forming first coatinglayer 20 include wet coating techniques such as dipping and spraying.For dip coating, a portion of the surface of substrate 10, including theentire surface, is immersed in an alkoxide solution in the presence of acatalyst and alcohol. The alkoxide may then undergo hydrolysis andcondensation reactions to form a network (e.g., a polymeric network) offirst control layer 20 on the unmersed surface.

Referring to FIG. 1, on first coating layer 20 (top surface as viewed)is second coating layer 30. In one embodiment, second coating layer 30is a material that promotes osseointegration between substrate 10 andbone material (e.g., human bone material). One suitable material is acalcium phosphate material such as hydroxyapatite (HA). In oneembodiment, second coating layer 30 is a layer including crystalline HAnanoparticles (e.g., having an average particle size on the order of anaverage particle size of 10 to 100 nanometers). One source of HAnanoparticles is Berkeley Advanced Biomaterials, Inc. of San Leandro,Calif. Representatively, BABI-HAP-N20™ has a particle size on the orderof 20 nanometers.

The HA nanoparticles may be introduced on a surface of substrate 10 (onfirst coating layer 20) in the form of a colloid. To form a colloid,nanoparticles of HA may be combined (dispersed) in solution with asolvent and mixed at a pH on the order of 7 to 10. A representativeamount of HA in a colloid is on the order of 0.15 weight percent.

The HA nanoparticles of second coating layer 30 have a property thattends to ionically-bond to first coating layer 20, particularly topositively-charged ligands of first coating layer 20. The ionic bondingcreates a “self-assembled” coating (a combination of first coating layer20 and second coating layer 30). A static charge attraction may becreated on a surface of second coating layer 30 between thepositively-charged ligands and appositively charged nanoparticles. UsingHA nonoparticles as a material of second coating layer 30, a thin layer(e.g., a monolayer) has strong adhesion properties that promoteosseointegration.

To form a coated substrate such as illustrated in FIG. 1, the substratesurface is initially thoroughly cleaned and charged (e.g., negativelycharged) through chemical processing. First coating layer 20 is thenapplied to a portion of the surface substrate 10 by dipping or other wetcoating process. Curing of first coating layer 20 may be done at roomtemperature in a matter of minutes. In the embodiment where firstcoating layer 20 is derived from (APS), a monolayer can provide auniform one-dimensional distribution of cations is formed at theoutermost surface of first coating layer 20. Substrate 10 includingfirst coating layer 20 is then dipped into a solution including, in oneembodiment, a colloid of HA, to form second coating layer 30 (e.g., anHA anionic layer). Representatively, substrate 10 may be immersed in anHA colloid solution for several minutes (e.g., 10 minutes) then rinsedand cured. Second coating layer 30 may be cured at a temperature of roomtemperature to about 100° C. To form additional layers of second coatinglayer material (e.g., multiple HA layers), the process may be repeated.

A representative thickness for second coating layer 30 of one or more HAlayers is on the order of 10 nanometers (e.g., one layer) to 100nanometers. The multilayer coating (e.g., first coating layer 20 andsecond coating layer 30) thus formed can have a negatively chargedsurface. This ionic self-assembled multilayer coating or film also canbe synthesized on virtually any shape implant and can provide aconformal coating on a rough metal (e.g., titanium) surface that may bemaintained with the coating layer.

In one embodiment, the method applies sol-gel processing in aself-assembled method to produce a nanometer scale of calcium phosphate(e.g., HA) ultrathin-coating at room temperature to 200° C. Acrystalline HA coating formed as described possesses high bioactivityand biocompatibility. The electrostatic nature of the coating improvesadherence of an HA film to implant surfaces. The adherence is believedto be due to a chemically-bonded first or primary coating layer adjacentthe metal implant and a second layer ionically bonded to the firstlayer. In one embodiment, a process produces a multilayer coating on theorder of 10 nm to 200 nm thickness.

By using a calcium phosphate (e.g., HA) surface coating early andaccelerated osteoblast adhesion may be generated thereby reducing thehealing time during implantation, and should benefit patients withinadequate bones or significant load bearing implant designs, for whomno definite treatments are currently available. the coating describedmay also be non-toxic based on in vitro and in vivo assay. Results of anAPS (first coating layer) and HA (second coating layer) coating indicatethat genes associated with bone formation (col1, OPN, OCN) were equallyexpressed among the condition tested, while the implant specific geneswere upregulated in HA coated discs as much as 50 percent.

EXAMPLES Example 1 The Hydroxyapatite (HA)-Sol Preparation

HA nanopowder (˜20 nm) was used for formulation.

HA sol preparation: Materials: BABI-HAP-N20™ (100 g net weight in theammonia hydroxide), ether, 2-methoxyethanol, and dH₂O. Add 1 M NaOH andadjust the pH to 9-10. Move away the transparent solution on the top.Take the HA powder add in 2-methoxyethanol (3 percent). Ultrasonicallyagitate for 30 minutes at room temperature (RT) to make 3 percent HAcolloid.

Example 2 Self-Assembled Coating Preparation

Surface cleaning of Ti substrates: Preparation of Piranha solution: Add45 ml 30 percent H₂O₂ and then 105 ml 100 percent H₂SO₄ (3:7) in a glassbeaker of 200 ml. The resulting solution is divided to 3 parts and addin three glass bottles of 60 ml. Commercial implants are introduced intothe solutions respectively. The bottles are then placed in an oven at80° C. for 1 hour. Rinsing extensively with Milli-Q H₂O. Rinsing againwith absolute Ethanol. Drying in the oven.

APS modification of Ti surface: Preparation of 5 percent APS solution:Add 3-aminopropyltriethoxysilane in pure Toluene. Immerse the implantsin 5 percent APS solution for 15 hours at RT. Ultrasonically agitate for30 minutes each in toluene, methanol/toluene (1:1), and methanol.Rinsing extensively with Milli-Q water to remove residual APS. Dry atroom temperature (RT) for 1 to 2 minutes and store till use.

Alkaloid water: Add NaOH in Milli-Q water and adjust the PH to 10.

Example 3 Synthesis of Ultrathin Film of Hydroxyapatite

Immerse the APS-modified titanium implants into 3 percent HA solutionfor 10 minutes at RT. Rinse by alkaloid dH₂O. Cure at 100° C. for 30minutes.

Example 4 Cross-Cut Tap Test for Tensile Strength (Adhesion) of HaUltrathin Film on Titanium

The adhesion of HA ultrathin film on metal titanium is tested by aCross-Cut kit (Precision Gage & Tool Co., Ohio). Briefly, the specimenswere placed on a firm base, under room temperature. An area free ofblemishes and minor surface imperfections is selected, using thecross—cut tool from the lit to make parallel cuts. Make a second cut at90 degree to and centered on the original cuts to create grid in thefilm. Place the center of the test tape over the grid and smooth it intoplace. To make good contact with the film, rub the tape firmly with apencil eraser. Wait about 90 seconds, and then remove the tape. Seizethe free end and quickly pull it back upon itself as close as possibleto an 180 degree angle.

Using the kit's illuminated magnifier, inspect the grid area for removedcoating. The adhesion of the coating is rated according to the followingscale: 5B—none of the coating on the grid squares is detached; 4B—nomore than approximately of the area is detached; 3B—approximately 5 to15 percent of the area is detached. 2B: approximately 15 to 35 percentof the area is detached. 1B: approximately 35 to 65 percent of the areais detached. 0B: Flaking is worse than Grade 1B.

A grade of 5B was obtained for the coating formed as described inExamples 1-3 on the metal titanium and showed very strong adhesion ofthe coating.

Example 5 Test for Cellular Toxicity of Nano-Ha Coating In Vitro

A disk including a coating formed as described in reference to Examples1-3 was used for in vitro cell culture studies. Adult maleSprague-Dawley rats (approximately 100-150 g body weight) were used toisolate bone marrow stromal cells (BMSC). BMSC were divided in 4 groupsand cultured on top of turned disk, acid-etched disk, turned-HA coateddisk, and acid-etched-HA coated disk. BMSC were maintained in aconventional osteoblastic differentiation medium. BMSC proliferateduneventful on the HA surfaces and no significant difference were notedamong the groups during the experimental period, suggesting that thecoating formed as described do not exhibit any cellular toxicity. Theculture was terminated at day 14 and BMSC were harvested for total RNApreparation. The steady state expression of mRNAs encoding boneextracellular matrix proteins and implant associated genes wereevaluated by RT-PCR. FIG. 2 shows a RT-PCR evaluation of the steadystate mRNA levels of type I collagen (col1), osteopontin (OPN),osteocalcin (OCN), TO1, TO2, and TO3 in rat bone marrow stromal cells.The expression levels of these mRNA species were similar in thesegroups. FIG. 2 also shows a housekeeping gene, GAPDH that was used as aninternal control.

The results indicate that genes associated with bone formation (col1,OPN, OCN) were equally expressed among the condition tested, while theimplant specific genes were upregulated in HA coated discs as much as 50percent.

Example 6 Cell Adhesion Osseointegration In Vivo

The miniature Ti rods with or without HA coating were UV sterilized andimplanted in the femur of adult male Sprague-Dawley rats according tothe previously described method. At day 14, the femurs were harvestedand subjected to the implant push-in test. The HA coated titaniumimplant showed over a 200 percent increase of the push-in value ascompared with the uncoated group. The results are illustrated in FIG. 3.This finding suggests that an HA coated structure coated as described inExamples 1-3 may positively stimulate osseointegration. The HA coatedsurface roughness was similar to uncoated titanium implant surface(Table 1).

TABLE 1 Turned Turned-HA Rp-p 0.252 0.315 Rms 0.050 0.047 Ra 0.041 0.040

In the preceding paragraphs, specific embodiments are described. Itwill, however, be evident that various modifications and changes may bemade thereto without departing from the broader spirit and scope of theclaims. The specification and drawings are, accordingly, to be regardedin an illustrative rather than a restrictive sense.

1. A method comprising: chemically charging a surface of a medical or dental implant; and contacting the implant with a colloid comprising crystalline hydroxyapatite nanoparticles on the implant.
 2. The method of claim 1, wherein the implant comprises a titanium alloy.
 3. The method of claim 1, wherein prior to contacting the implant with a colloid of crystalline hydroxyapatite nanoparticles comprises: dispersing the crystalline hydroxyapatite nanoparticles in solution with a solvent.
 4. The method of claim 1, further comprising forming a positively-charged layer from a condensation reaction of an alkoxide on the surface of the implant and forming a layer of crystalline nanoparticles on the positively-charged layer.
 5. A method comprising: cleaning a surface of a medical or dental implant; and forming a layer of crystalline hydroxyapatite nanoparticles on the implant.
 6. The method of claim 5, wherein the implant comprises a titanium alloy.
 7. The method of claim 5, wherein forming a layer of crystalline hydroxyapatite nanoparticles comprises: contacting the implant with a colloid comprising the crystalline hydroxyapatite nanoparticles dispersed in solution with a solvent.
 8. A composition comprising: a plurality of negatively charged crystalline HA nanoparticles having an average particle size of 10 to 100 nanometers dispersed in a basic solution of mixed solvents.
 9. The composition of claim 8, wherein the mixed solvents comprise 2-methoxyethanol.
 10. The composition of claim 8, comprising a pH of 9-10.
 11. The composition of claim 8, wherein the average particle size comprises 20 nanometers. 